Solar cells convert light into useful energy, such as electricity or chemical energy. The high cost of solar cells, however, has prevented them from competing with conventional devices for generating power. Solar cells have typically been limited to low power applications, such as calculators, or niche applications, such as powering spacecraft, buoys, or other remote equipment.
Solar cells are typically constructed by forming a pn junction on a wafer of single crystal, electronic grade semiconductor silicon. The pn junction is typically formed parallel to the major surfaces of the silicon wafer. One side of the pn junction is electrically contacted by a conductor on the back surface of the solar cell, while the other side of the pn junction is contacted by a metallic grid on the front surface of the solar cell. Light incident on the cell creates electron-hole pairs that cause a voltage difference between the conductor on the back surface of the cell and the conductive grid on the front surface of the cell. Because such cells require electronic grade semiconductor silicon, they are expensive to manufacture. Such cells are relatively fragile and typically require mounting in a protective enclosure having a cover of a translucent material, i.e., a material that transmits a portion of the incident light.
Another type of solar cell that is constructed from spheres of metallic grade silicon is described in Levine, et al., "Basic Properties of the Spheral Solar.TM. Cell," Proceedings of the Twenty Second IEEE Photovoltaic Conference, Vol. 2, pp. 1045-48 (1991). Spheres of metallic grade silicon somewhat smaller than 1.0 mm in diameter and including a p-type dopant are purified, and an outer shell of each sphere is doped with an n-type material to form a pn junction. The spheres are bonded to a flexible aluminum foil, and electrical contacts are formed between the aluminum and the outer n-type shell. The spheres are etched to allow formation of an electrical contact to the inner p-type material. Although such cells are purportedly cheaper to produce than cells using wafers of electronic grade semiconductor silicon, the manufacture of such cells is complex. Furthermore, such cells, like previous cells, are relatively fragile and must be mounted in a protective module having a translucent glass or polymer superstrate.
Another type of solar cell uses silicon crystals embedded in a frit glass insulator, surrounded by clear hydrobromic acid. The voltage across the silicon crystals causes an electrochemical reaction that produces gaseous hydrogen, liquid bromine, and heat. The solar energy is thus stored as chemical energy in the hydrogen and bromine, which can be used in a fuel cell. McKee, et al., "Development and Evaluation of the Texas Instruments Solar Energy System," 16th IEEE PVSC Proceedings, p. 257 (1982).